This is a double-blind, randomized, multicenter trial for the prevention of CMV end-organ disease in HIV-infected patients with CD4+ lymphocyte counts <100 cell/fL. Patients will be randomized to BW256U87, high-dose acyclovir, or low-dose acyclovir. All groups will also take matching placebos. The acyclovir control groups will provide suppressive therapy for Herpes simplex infections and allow us to investigate a possible dose-dependent effect on survival.